a) Field of the Invention
This invention relates to the treatment of volatile contaminants. The invention is particularly suitable for, but not limited to, the removal of contaminants from solids and liquids.
The contaminants may include, but are not limited to, petroleum products (eg. petrol, oils, greases); phenols; coaltar; cyanide; pesticides; PCB""s; HCB""s, organochlorine pesticides and arsenics.
The treatment of contaminated soils and liquid wastes is a worldwide problem. Often, the contaminated soils or liquids are simply removed and transferred to a toxic waste dump or pond. This does no more than move the problem. For contaminants such as PCB""s, the environmental protection authorities around the world specify strict conditions for their disposal in very high temperature incinerators, eg. found in the vessel xe2x80x9cVulcanusxe2x80x9d.
b) Description of the Prior Art
International Patent Application No. PCT/AU93/00646 (International Publication No. WO 94/15150) (Robertson) discloses a stationery retort where toxic waste and other contaminants are removed from soil, the soil being agitated and being brought into contact with the retort walls to cause the wastes and contaminants to be desorbed. The retort has proved successful in the removal of toxic waste and contaminants from many types of soil.
It is an object of the present invention to provide improved methods and apparatus for use in removing volatile contaminants from solids or liquids.
According to one aspect of the present invention there is provided a method for the treatment of volatile material(s) in contaminated material(s) including the steps of:
feeding the contaminated material(s) to a retort assembly which includes a rotatable retort at least partially disposed within a combustion chamber which is heated by heating means;
causing the contaminated material(s) to contact the wall(s) of the retort to cause the volatile material(s) to be given off as gases;
discharging the treated material from the retort;
transferring the gases to an afterburner for combustion; and
returning the combustion gases from the afterburner to the retort assembly to provide assistance in heating the contaminated material(s) being treated in the retort.
According to another aspect of the present invention there is provided apparatus for the treatment of volatile material(s) in contaminated material(s) including a retort assembly which includes a rotatable retort disposed at least partially within a combustion chamber with heating means to indirectly heat the rotatable retort; said rotatable retort include a feed end through which the contaminated material(s) are fed to the retort and a discharge end from which the materials are discharged from the retort; an afterburner; means to transfer the volatile material(s) given off as gases to the afterburner for combustion; and means for passing the combustion gases from the afterburner to the retort assembly to provide additional heat for use in the heating of contaminated material(s) in the retort.
Preferably, the apparatus includes a high temperature filter through which the gaseous volatile material(s) pass after leaving the retort and prior to entering the afterburner.
According to yet another aspect of the present invention there is provided a high temperature filter which is suitable for use but not limited to the treatment of volatile gaseous contaminated material, the filter including a main body having first and second chambers therein which chambers when the filter is in its operative position include an upper region and a lower region, an opening providing communication between the chambers, said opening being at the lower region of the chambers, an inlet for delivering gaseous contaminated material to the first chamber in the upper region thereof, an outlet for discharging the gaseous material from the second chamber, a solids collection zone adjacent the opening, a solids discharge outlet for discharging solids from the solids collection zone, a baffle opposite to and spaced from the inlet upon which incoming gases impinge and filter means for filtering the gaseous material passing out of the second chamber via the outlet.
According to another aspect of the present invention there is provided a retort for use in the treatment of volatile material, the retort including a cylindrical body which is mounted for rotation about its longitudinal axis, said body having an infeed end and an outlet end, a combustion chamber, said cylindrical body being at least partially located within the combustion chamber, a plurality of balls or like elements disposed within the cylindrical body arranged to interact with contaminated material when the cylindrical body is rotating to break down the material and dislodge carbonised material which may form on the internal wall of the cylindrical body. The retort is particularly suitable for use in apparatus of the type described herein.
Preferably the retort includes a cage within the cylinder which retains the balls in the region of the wall of the cylindrical body. Preferably, the balls are arranged in groups, the groups being at spaced intervals along the interior of the cylindrical body. The cage includes spaced apart peripherally extending members which are adapted to assist in retaining the balls in each group at a particular location within the cylindrical body. Preferably, the balls are made of ceramic material. Preferably, the cage is mounted for rotation in the opposite direction to the cylindrical body.
In one embodiment of the invention the combustion gases are passed through the interior of the retort. In another embodiment the combustion gases are passed to the heating means.
When the contaminated material to be treated is in the form of solids, the solids are preferably passed through a grizzly or sieve prior to entering the retort to remove oversized material. If desired the solids may in addition to or alternatively to the above be passed through a mill prior to entering the retort so as to reduce the particle or granule size of the solids.
When the contaminated material is in the form of liquid, the water content of the liquid is preferably reduced prior to entering the retort. To this end the liquids may be preheated to boil off the water prior to entering the retort.
Preferably, the rotary retort rotates about an axis inclined at a small angle to the horizontal and is substantially surrounded by a combustion chamber to enable indirect heating of the retort
Preferably, the combustion gases pass through a scrubber before being released into the atmosphere after passing through the retort. The gases from the high temperature filter may be passed through a condenser, where the condensate contains hydrocarbon fractions such as fuel oil and lubrication oil fractions.
In the high temperature filter according to the invention, the baffle is preferably defined by a wall which separates the said first and second chambers. Preferably, the wall extends from an upper internal wall of the chambers and terminates at a point spaced from a lower internal wall of the chambers, the space between the free end of the wall and the lower internal wall of the filter defining the opening. The wall may have fins thereon.
Preferably, the discharge outlet comprises a plurality of outlet ports in the upper wall of said second chamber. Preferably, the filter means comprises a plurality of ceramic candles, each ceramic candle being associated with a respective outlet, the ceramic candles extending into the second chamber.
There may further be provided a gas collecting chamber for receiving the gaseous material from the outlets and a discharge outlet for discharging the gaseous material from the gas collecting chamber. Fan suction means may be provided for drawing the gaseous material from the second chamber through the outlets.
The filter may further include pulsing means for delivering a gas under pressure to the filter means in the opposite direction of normal flow for cleaning the filter means. Preferably, the gas used in the pulsing means is nitrogen.
A heat jacket is preferably provided which at least partially surrounds the main body of the filter.
A further embodiment of the invention is particularly suited for the treatment of material containing organochlorine pesticides such as DDT, DDE and DDD and various arsenic based compounds. Such material is found in soil from cattle dip sites.
According to this aspect of the present invention there is provided a method for treatment of volatile material(s) in contaminated material(s) including organochlorine pesticides and arsenic based compounds including the steps of:
feeding the contaminated material(s) to a retort assembly which includes a rotatable retort at least partially disposed within a combustion chamber which is heated by heating means;
causing the contaminated material(s) to contact the wall(s) of the retort to cause the volatile material(s) to be given off as gases;
discharging the treated material from the retort into a high temperature filter;
thereafter transferring the gases to an afterburner for combustion and at the same time introducing water vapour into the afterburner.
According to yet another aspect of the present invention there is provided apparatus for treatment of volatile material(s) in contaminated material(s) including organochlorine pesticides and arsenic based compounds including
a retort assembly which includes a rotatable retort having an infeed end through which material is fed to the retort and an outlet, the retort being at least partially disposed within a combustion chamber which is heated by heating means, whereby in use, the contaminated material(s) is caused to contact the wall(s) of the retort to cause the volatile material(s) to be given off as gases;
a high temperature filter which receives the material from the retort, an afterburner for combustion of the gases and means for introducing water vapour into the afterburner.
In this particular process the contaminated material is preferably firstly pretreated to remove water from the material. This may be done by the use of a preheater. The material is then transferred to a retort where the contaminant compounds are vaporised. The contaminants in the gas stream so formed are then transferred to a high temperature filter which may be of the type described earlier where further particulate matter is separated from the gas. The remaining gaseous component is transferred to an afterburner. The afterburner thermally destructs the organochlorine pesticides to produce simple products of combustion and hydrogen chloride gas. The arsenic component of the gas will pass through the afterburner primarily as arsenic trioxide.
If desired water vapour which may be conveniently drawn from the preheater is fed into the afterburner. The introduction of the water vapour causes a water/gas reaction which assists in the production of hydrogen chloride and arsenates.
The gas stream then passes to a condenser wherein the gas is rapidly cooled so as to condense the arsenates for collection as particulate matter. After the gas stream leaves the condenser calcium carbonate can be added to the stream to neutralise the hydrogen chloride.
The gas can be finally passed through a dust collector device whereafter the gas can pass to atmosphere.
As mentioned earlier the gases leaving the afterburner are cooled so as sublimate (condense) the arsenic and arsenic trioxide. Two alternative systems are envisaged
1. indirect air cooled; or
2. evaporative cooling through injection of water into the gas stream.
The gas stream leaving the afterburner is cooled to preferably about 110xc2x0 C. in the condenser and then may be dosed with calcium carbonate (CaCO3) (lime). The calcium carbonate reacts with the constituents of the gas stream to neutralise the hydrochloric acid and absorb moisture in the gas stream. The lime assists in minimising moisture problems on the filter bags, and can be collected in a dust collection bin.
The dust collector which may be in the form of a baghouse will remove the particulate arsenic trioxide which condenses below at approximately 120xc2x0 C. and collects on the filter media. The gas stream will exit the baghouse at approximately 100xc2x0 C. and be vented to atmosphere. An auxiliary fan on the baghouse will be used in conjunction with the high temperature filter fan in order to overcome the additional pressure loss in the system. The fans will be balanced using dampers in the system. The contaminated particulate (arsenic trioxide, spent lime) can be collected in plastic lined 200 L drums for disposal at authorized landfills.